In refrigeration systems such as those used for cooling food cases in grocery stores, it is necessary to maintain a constant temperature in the cases to ensure the quality of their contents. Shopping patterns, items being on sale, the cleaning and stocking of the cases all contribute to varying the cooling load on the evaporators cooling the respective cases. To create variable capacity control and to maintain tight tolerance in suction pressure or refrigerated space temperature, it is conventional to control capacity by cycling or unloading compressors. Both methods cause the suction pressure to fluctuate and consequently the refrigerated space temperature varies. The use of inverter drives is effective but very expensive. Additionally, two-stage internally compounded compressors (compound cooling compressors) cannot easily be applied with cylinder unloading. Where an economizer is used, the thermal expansion valve (TXV) is controlled by interstage and/or discharge temperature.